1. Field of the Invention
This invention relates to phase comparison radio navigation systems of the kind in which spaced transmitting stations radiate phase-locked signals of the same frequency in a time shared sequence and in which, at a receiver, the signals from the spaced stations are compared in phase.
2. Prior Art
It is known in such a system to have one of the transmitting stations as a prime or master station and to provide the other transmitting stations, referred to as secondary or slave stations, with means for receiving signals from the prime station to determine or control the frequency and phase of the radiation from the secondary station. Heretofore, in such systems, the secondary stations have had a receiving antenna for receiving the signals from the prime station, an oscillator and a phase discriminator comparing the received prime signals with the oscillator output to maintain the oscillator in the required frequency and phase relation with the prime signals. The oscillator output is then used to drive a power amplifier feeding a transmitting antenna to provide the radiated secondary signals. A further phase discriminator and phase lock loop is provided to compare the phase of the radiated secondary signals with the oscillator output and to control a phase shifter for adjusting the phase of the radiated signals so as to compensate for any variations in phase shift between the oscillator and the antenna radiation. When separate receiving and transmitting antennae are employed in this way, it is possible for changes in ground conditions to modify the coupling between the antennae and hence to alter the relationship between the transmitted signals and the incoming datum.